cradit.cpp

#include <iostream>
using namespace std;

int main() {
    long long number;
    cin >> number;

    int oddSum = 0;
    int evenSum = 0;

    bool even = false;

    bool visaHead = false;
    bool mastercardHead = false;
    bool americanExpressHead = false;

    int i = 0;

    while (number > 0) {
        i++;
        int now = number % 10;

        if (even) {
            // 偶數位要做的事
            
            // *2
            now *= 2;

            // 個位數+十位數
            // 加到 evenSum
            evenSum += (now % 10) + (now / 10);

        } else {
            // 奇數位要做的事

            // 加到 oddSum
            oddSum += now;
        }
        
        number /= 10;

        if (number >= 10 && number <= 99) {
            if (number >= 40 && number <= 49) {
                visaHead = true;
            } else if (number >= 51 && number <= 55) {
                mastercardHead = true;
            } else if (number == 34 || number == 37) {
                americanExpressHead = true;
            }
        }

        even = !even;
    }

    // oddSum + evenSum
    int answer = oddSum + evenSum;

    // 判斷尾數是不是 0
    if (answer % 10 != 0) {
        cout << "無效卡號";

        return 0;
    }
    
    if (visaHead && (i == 13 || i == 16)) {
        cout << "VISA";

        return 0;
    }

    if (mastercardHead && i == 16) {
        cout << "MasterCard";

        return 0;
    }

    if (americanExpressHead && i == 15) {
        cout << "American Express";

        return 0;
    }

    // return 0;
}